1. Field of the Invention
The present invention concerns coding numbers in binary form using a pseudo-logarithmic compression law providing, for the same number of binary digits in the code word, a coding range (that is to say, a set of codable number values) that is more extensive than natural binary at the cost of reduced accuracy, which is no longer one hundred percent, the same code word value being assignable to several neighboring number values.
2. Description of the Prior Art
The best known binary codes of this kind are the so-called A law and .mu. law PCM codes defined in CCITT Recommendation G.711 which utilize a logarithmic compression law approximated by curves comprising 16 straight line segments with slope ratio equal to 2. Their eight-digit code words comprise three distinct parts, from left to right:
a sign - determining first part consisting of a sign bit, PA1 a segment - determining second part consisting of three binary digits serving in combination to identify one of eight segments, and PA1 a third part determining the interval within the segment, consisting of four binary digits serving to determine the interval concerned within the segment. PA1 said lefthand part of the code word is made up of a variable number p of binary digits of the same value (hereinafter called 1), the number p corresponding to the rank number of the compression law curve segment concerned as counted from the origin and chosen as equal to the leftward shift, relative to the position of the most significant bit of the code word, of the most significant bit of the number to be coded, expressed in natural binary and augmented by 2.sup.n, and, PA1 said righthand part of the code word, when present, is made up of a binary number having at the position of the most significant bit a separator bit the value of which (hereinafter called 0) is the complement of that of the said p binary digits of said lefthand part and corresponds to the expression in natural binary of the number to be coded augmented by 2.sup.n and then truncated on the right by 2p digits and deprived of its most significant bit. PA1 a divider circuit with a plurality of division ratios operating on said series of counting pulses and having one output corresponding to a division ratio of 2.sup.n and another output corresponding to a variable division ratio selected from powers of 4 corresponding to the rank numbers of the segments of the compression law as counted from the origin, the first segment having the rank number 0, PA1 a synchronization and selector circuit which determines, by counting packets of 2.sup.n counting pulses sent by the divider circuit and by registering when the number of said packets crosses a threshold 2.sup.i -1 where i is an integer between 1 and n, when lower thresholds of subranges corresponding to the segments of the compression law curve with rank number i are crossed and which commands said divider circuit to select the division ratio corresponding to the ith power of 4 and which resynchronizes said divider circuit after a subrange lower threshold has been crossed, and PA1 an n+1 stage counter circuit adapted to count pulses delivered by the variable division ratio output of said divider circuit and to yield a count which gives the value of the code word corresponding to the number of counting pulses applied to said divider circuit.
The A and .mu. laws follow compression laws for which all segments except the first two comprise 16 intervals the amplitude of which doubles from segment to segment. Law A has coincident first and second segments with 32 intervals of amplitude 2 which gives a coding range running from -4 095 through +4 095 and a maximal coding error, outside the first segment, running from 4.7% for the second segment through 6.2% for the last segment. The law .mu. has a first segment with 16 intervals of amplitude 2 except for the first which has amplitude 1 and a second segment of 16 intervals of amplitude 4, which gives it a coding range extending from -8 158 through +8 158 and a maximal coding error, outside the first segment, running from 9.7% for the second segment through 6.3% for the last segment.
These A law and .mu. law PCM codes are well suited to digital coding of speech but have disadvantages in other applications such as the transmission of error rates in equipment distributed along a digital transmission link. These error rates, which are expressed as numbers of errors detected per surveillance cycle, occupy a wider range than the coding range of the A law and .mu. law PCM codes, even if modified given the lack of need for the sign bit, and necessitate a non-uniform accuracy of coding, high at the bottom end of the range and just sufficient at the top end of the range to indicate the orders of magnitude of high error rate values. It is important to have a high accuracy of coding for a low value error rate which may occur when the link is in a functioning state in order to be able to identify from a supervised terminal isolated, regular or random errors and to monitor the quality of functioning of equipment distributed along the link sufficient accurately to establish a maintenance program with preventive servicing to avoid interruptions of traffic, while knowing the order of magnitude concerned or that a range overshoot has occurred in the case of high error rates facilitates identification of the faulty equipment.
It is possible to enhance the accuracy of the A law and .mu. law PCM codes by doubling the number values before coding them. This introduces a first subrange (0-31 for the A law) in which the coding accuracy is total but at the cost of reducing the total coding range by one half (0-2 047 for the A law), this range being already insufficient in the aforementioned applications.
In these aforementioned applications it is also important that coding be easy to implement using counters, the number of errors detected per surveillance cycle which constitutes the error rate being originally available in the form of counting pulses.
One object of the present invention is a method of coding numbers in binary form using a pseudologarithmic compression law which has enhanced characteristics as compared with the A law and .mu. law PCM codes both with regard to the accuracy at the bottom end of the range and the total extent of the coding range, and which is well suited to transmission of error rates of equipment distributed along a digital transmission link.
Another object of the invention is a method of coding numbers in binary form using a pseudo-logarithmic compression law which can be implemented on the basis of numbers available as counting pulses using a simple and reliable counting device that can form part of an equipment that is relatively inaccessible, as is generally the case with the equipment distributed along a digital transmission link.